In recent years, attention has been focused on the use of big data, and traffic volume and data processing volume in data centers are increasing. Correspondingly, the data volumes and the processing capabilities of information communication devices are increasing. At that time, the communication speeds inside and outside the devices which are a bottleneck of large-scale data processing are also improving. For example, in communication outside the device, a transmission rate of 56 Gbps per channel is being standardized.
It is thought that communication is made difficult with a conventional non return to zero (NRZ) modulated signal because high-speed communication causes an increase in transmission loss in a communication path and 1 unit interval (UI) width of data is reduced. As described above, there is a limit to high-speed communication in a time direction, and it has been proposed to use pulse amplitude modulation-4 (PAM) which is a type of multi-level modulation, for a generation of 56 Gbps transmission rate per channel (see NPL 1).